Rotational energy absorber and fall arrest system

ABSTRACT

A rotational energy absorber typically for use in a fall arrest system has a coiler, a length of plastically deformable strip and a deformer structure. The plastically deformable strip has a first end attached to the coiler and a second free end and extends past the deformer structure at a position between the first and second ends. Relative rotation of the coiler member and deformer structure causes the strip to be drawn past the deformer structure, plastically deforming the strip and winding the strip coil form about the coiler member.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved rotational energy absorber and inparticular to an improved rotational energy absorber for use in a fallarrest system.

2. State of the Art

Fall arrest systems are used to prevent personnel working at a heightfrom suffering injury or death due to falls. Fall arrest systems arealso often referred to as height safety systems or fall preventionsystems.

A common type of rotational energy absorber used in fall arrest systemsis a friction brake type energy absorber. This generally comprises twocircular, or annular, friction pads, held in face to face contact with apreset load between them. When a sufficiently large couple is appliedbetween the two pads, relative rotation of the two pads will take placeand the friction between the pads will absorb energy. In a fall arrestsystem the rotational energy absorber is used to absorb the kineticenergy of a falling user, so slowing and then arresting their fall.

There are a number of problems with such frictional rotational energyabsorbers. Firstly, there is the fundamental problem that the appliedcouple required to start relative rotation of the two pads is largerthan the couple required to continue rotation. This is a particularproblem in a fall arrest system because there is a maximum safe loadwhich a falling person can be subjected to while their fall is beingarrested. The fall arrest system must be arranged so that the loadapplied to a falling user by the couple required to start relativerotation of the friction pads is not larger than the safe limit, so itfollows that the load applied to the falling user by the smaller couplerequired to continue relative rotation of the friction pads will be lessthan the safe limit. As a result the rate at which the energy of thefall is absorbed is less than could be achieved if the load applied tothe falling user was constant at the safe limit, so that the fallinguser will fall further before their fall is arrested and the userbrought to a stop. The extra distance fallen increases the risk that thefalling user will be injured by impact with obstacles, or the ground.Further, the extra distance fallen increases the total amount of energywhich must be absorbed, requiring an increase in the energy absorbingcapacity of the energy absorber, and thus an undesirable increase insize, weight and cost.

Further, the degree of friction between the two pads is dependent uponthe contact load acting between the two pads. As a result, in order toset the couple required to start and continue relative rotation of thepads to a desired value, such frictional energy absorbers requireprecise setting of the contact load. Further, the mechanism used to setthe contact load, generally some form of spring, suffers from theproblem of the applied load changing over time due to environmentaleffects.

Finally, the couples required to start and continue the relativerotation of the pads is highly sensitive to the surface properties ofthe pads. These properties are subject to the problem of the requiredcouple changing over time due to environmental effects.

SUMMARY OF THE INVENTION

The present invention was made in order to overcome these problems, atleast in part.

In a first aspect, the present invention provides a rotational energyabsorber comprising:

-   -   a coiler member;    -   an elongate element of a plastically deformable material; and    -   a deformer structure;    -   the elongate element having a first end attached to the coiler        member and a second free end remote from the first end; the        elongate element extending past the deformer structure at a        position between the first and second ends; relative rotation of        the coiler member and deformer structure causing the elongate        element to be drawn past the deformer structure, plastically        deforming the elongate element and winding the elongate element        coil form about the coiler member.    -   In a first embodiment of the invention, the coiler member may        rotate with respect to a relatively fixed deformer structure.

The coiler member may comprise an inner member arranged for relativerotation with respect to an outer member; the outer member comprisingthe deforming structure. Beneficially, actuation is such that when acouple below a predetermined value is applied to the coiler member, theelongate element is not drawn past the deformer structure and there isno rotation of the coiler, and when a couple above the predeterminedvalue is applied, the elongate member passes the deformer structure andis plastically deformed, so absorbing energy and permitting rotation ofthe coiler member and the outer member.

In a second aspect, the invention provides a rotational energy absorbercomprising an inner member and an outer member arranged for relativerotation and an elongate element of plastically deformable material; theouter member comprising a deforming means; the elongate element having afirst end attached to the inner member and a second free end remote fromthe first end, and passing through the deforming means at a pointbetween the first and second ends; so that when a couple below apredetermined value is applied between the inner member and the outermember the elongate element does not pass through the deforming meansand there is no relative rotation of the inner member and the outermember, and when a couple above the predetermined value is appliedbetween the inner member and the outer member the elongate member passesthrough the deforming means and is plastically deformed, so absorbingenergy and permitting relative rotation of the inner member and theouter member.

This provides the advantage that because the predetermined value of theapplied couple required to deploy the elongate element is determined byplastic deformation and not friction, the applied couple required tobegin deployment of the elongate element and operation of the energyabsorber is the same as the applied couple required to continue thedeployment and operation. Accordingly, when used in a fall arrest systemthe deployment of the elongate element can be carried out throughout theoperation of the energy absorber at a level of applied couplecorresponding to the maximum safe load which can be applied to the user,so minimising the length of the fall before the user fall is arrested.

Further, because the predetermined value of the applied couple requiredto deploy the elongate element is determined by the plastic deformation,and thus the bulk material properties, of the elongate element and notthe surface properties of a component, the energy absorber is lesssubject to change over time due to environmental effects.

Further, the predetermined value of the applied couple required todeploy the elongate element is determined by the properties of theelongate element and the deforming means and not by any externallyapplied load. Accordingly, there is no requirement for setting of aprecise contact load or any problem of changes in a contact load overtime due to environmental effects.

Preferably, a space is defined within which the elongate element isstored, in coil form, after passing through the deformer structure, thespace being insufficiently large to contain all of the elongate element,so that the space will become filled with the elongate element and theelongate element will be stopped from passing through the deformerstructure before the free end of the elongate element passes through thedeforming means.

This provides the advantage that deployment of the elongate element isbrought to a stop in a controlled manner before the elongate element isreleased from the deformer structure.

According to a further aspect, in simplified form, the invention maycomprise a rotational energy absorber comprising:

-   -   a coiler member;    -   an elongate element of a plastically deformable material;    -   the elongate element having a first end attached to the coiler        member and a second free end remote from the first end, relative        rotation of the coiler member causing the elongate element to be        wound coil form about the coiler member, plastically deforming        the elongate element, such that following operation of the        energy absorber, the elongate element is wound coil form about        the coiler member.

Specific embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying diagrammatic figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of a first rotational energy absorberaccording to the invention;

FIG. 2 shows a schematic view of a fall arrest system incorporating theenergy absorber of FIG. 1;

FIG. 3 shows a schematic view of a second energy absorber according tothe invention;

FIG. 4 shows a schematic view of a third energy absorber according tothe invention in an initial condition;

FIG. 5 shows a schematic view of a the energy absorber of FIG. 4 in asubsequent condition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A rotational energy absorber 1 according to the invention is shown inplan view in FIG. 1. The energy absorber 1 comprises an axle 2 and acircular annular deformer ring 3 arranged concentrically about the axle2 and having an inner surface 3 a and an outer surface 3 b. The energyabsorber 1 is arranged to absorb energy in response to relativerotational movement of an axle 2 and a ring 3. In the illustratedembodiment the energy absorber is arranged to absorb energy in responseto anti-clockwise rotation of the axle 2 relative to the ring 3.However, it would be possible to arrange the energy absorber 1 to absorbenergy in response to clockwise rotation of the axle 2 relative to thering 3 by forming the energy absorber 1 as a mirror image of theillustrated embodiment.

The energy absorber 1 further includes a stainless steel strip 4 havinga constant thickness and width along its entire length. In operation,the energy absorber 1 absorbs energy by plastic deformation of thestainless steel strip 4.

A coiler member 5 is mounted on the axle 2 so that the coiler 5 rotateswith the axle 2 relative to the ring 3. An annular space 8 is definedbetween the coiler 5 and the inner surface 3 a of the ring 3. The ring 3has a deformer slot 3 c passing through the ring 3 and orientedtangentially to the inner surface 3 a of the ring 3. The slot 3 c isslightly wider than the thickness of the strip 4. The slot 3 e has aflat clockwise face 3 d and a parallel flat anticlockwise face 3 e, andthe anticlockwise face 3 e of the slot 3 is tangential to the innersurface 3 a of the ring 3. The slot 3 c has a curved entry surface 3 fwhere the clockwise surface 3 d of the slot 3 e contacts the outersurface 3 b of the ring 3 and a curved exit surface 3 g where theclockwise surface 3 d of the slot 3 c contacts the inner surface 3 a ofthe ring 3. The curved entry surface 3 f and the curved exit surface 3 gare separated by the flat clockwise face 3 d.

The strip 4 has a first end 4 a secured to the coiler 5 and a free end 4b. The strip 4 passes through the slot 3 e, and rest of the strip,ending in the free end 4 c, is coiled around the outer surface 3 b ofthe ring 3. An outer circular cover 6 is provided concentrically aboutthe axle 2 and ring 3 so that an annular strip storage channel 7 isformed between the outer surface 3 b of the ring 3 and the cover 6. Thecover 6 does not have any direct effect on the operation of the energyabsorber 1, but provides protection against environmental effects suchas weathering or dust.

In operation of the energy absorber 1, when the couple on the axle 2relative to the ring 3 reaches or exceeds a predetermined deploymentvalue the axle 2 and coiler 5 will rotate relative to the ring 3. Thisrelative rotation will wind the stainless steel strip 4 around the wheel5 and pull the stainless steel strip 4 coiled in the annular stripstorage channel 7 through the deformer slot 3 c into the annular space8. As the strip 4 passes through the slot 3 the strip 4 is plasticallydeformed and so absorbs energy.

As the strip 4 passes through the slot 3 b, the strip 4 is plasticallydeformed for a first time as the strip 4 is bent around the curved entrysurface 3 f, absorbing energy. The strip 4 is then plastically deformedfor a second time as it is bent around the curved exit surface 3 g,absorbing more energy. The strip 4 is further deformed, absorbing moreenergy, as the strip 4 is wound around the coiler 5, but this is arelatively small effect compared to the deformations and energyabsorption as the strip 4 passes through the slot 3 c.

In use, when the energy absorber 1 is subject to a couple between theaxle 2 and the ring 3 below a predetermined deployment couple the strip4 does not deform or pass through the slot 3 e so that there is norelative rotation of the axle 2 and the ring 3 and the energy absorber 1essentially behaves as a rigid body. When the energy absorber 1 issubject to a couple at or above the predetermined deployment couple thestrip 4 deforms and passes through the deformer slot 3 c, so absorbingenergy and allowing relative rotation of the coiler 5 (mounted on axle2) and the ring 3. If the couple drops below the predetermineddeployment couple the strip 4 will stop deforming and passing throughthe slot 3 c and the relative rotation of the axle 2 and the ring 3 willstop.

If the energy absorber 1 remained subject to an applied couple greaterthan the predetermined deployment couple the entire length of the strip4 would pass through the slot 3 c and when the free end 4 b of the strip4 passed through the slot 3 c the axle 2 (and coiler 5) would bereleased to rotate freely relative to the ring 3 without any energybeing absorbed. In order to prevent this, the annular space 8 definedbetween the coiler 5 and the inner surface 3 a of the ring 3 is arrangedto be too small to contain the full length of the strip 4 when the strip4 is wound around the coiler 5.

As a result, as deployment of the strip 4 continues and the strip 4passes through the slot 3 c into the annular space 8 and is wound aroundthe wheel 5, the strip 4 eventually fills the annular space 8 so thatthe different coils of the strip 4 wound around the coiler 5 are broughtinto contact and compressed together between the coiler 5 and the innersurface 3 a of the ring 3. This contact and compression results infriction between the different coils of the strip 4 and the wheel 5 andinner surface 3 a of the ring 3, increasing the applied couple requiredto continue deployment of the strip 4. This contact and compression ofthe strip 4 increases as more of the strip 4 passes through the slot 3 cand into the annular space 8 so that the amount of friction and therequired couple also increase, and as a result the deployment of thestrip 4 and the relative rotation of the axle 2 and the ring 3 will bestopped before the free end 4 b of the strip 4 passes through the slot 3c.

Thus, the total amount of energy absorbed by the energy absorber 1 iscontrolled by the size of the annular space 8 and length of the strip 4which can fit into the annular space 8.

The deployment couple is determined by the resistance to plasticdeformation of the strip 4 and not by frictional forces. As a result,the deployment couple required to start relative rotation of the axle 2and the ring 3 and the deployment couple required to continue relativerotation of the axle 2 and the ring 3 are the same. In order to reducefrictional forces affecting the deployment load as far as possible, andso keep the deployment couple required to start and to continuedeployment of the strip 4 constant the strip 4 is coated with surfacelayer of friction reducing material.

The deployment couple which must be applied between the axle 2 and thering 3 in order to deform the strip 4 so that the strip 4 begins to passthrough the slot 3 c can be set to any desired value by adjusting thematerial, width and thickness of the strip 4 and the shapes of the entrysurface 3 f and the exit surface 3 g.

An example of a fall arrest system 20 incorporating the energy absorber1 is shown in FIG. 2. The fall arrest device 20 comprises a length ofsafety line 21 wound on a drum 22. The drum 22 is connected to arespooling mechanism 24 which maintains a rewinding torque on the drum22 which acts to rewind any deployed length of safety line onto the drum22. Typically the respooling mechanism 24 is a clockspring type device.An attachment point 23 is used to connect the device 20 to a fixedsupporting structure (not shown). The drum 22 is also connected througha speed sensitive clutch 25 to the energy absorber 1.

In use, a user is attached to the free end of the safety line 21 bypersonal safety equipment, such as a safety harness. During normalmovement of the user, the safety line 21 is wound out from the drum 22to follow the movement of the user or any excess safety line 21 is woundback onto the drum 21 by the respooling mechanism 24. The speedsensitive clutch 25 is set not to respond to the slow rotation of thedrum 22 encountered during this normal movement of the user.

If the user falls, the safety line 21 is unwound rapidly from the drum22, resulting in rapidly accelerating rotation of the drum 22. When thespeed of rotation of the drum 22 reaches the response speed of the speedsensitive clutch 25 the clutch 25 engages the drum 22 to the energyabsorber 1. The load of the falling user is then applied to the energyabsorber 1 as a couple and the energy absorber 1 responds by deployingand absorbing energy until the fall is arrested.

In will be understood that if the strip 4 was entirely pulled throughthe slot 3 e in this example the user would then suffer an unconstrainedfall, and it is for this reason that it is important to ensure thatdeployment of the strip 4 is stopped before the free end 4 b of thestrip 4 passes through the slot 3 c.

The arrangement shown in FIG. 1 having the outer circular cover 6defining an annular storage channel 7 for the coiled strip 4 isadvantageous in order to provide a compact circular profile for theenergy absorber 1, which is particularly advantageous when the energyabsorber 1 is used in a fall arrest device 20 of the type shown in FIG.2. However, as explained above this is not essential.

Also, in the embodiment described the coiler member 5 is rotated whilstthe deformer ring 3 remains stationary. It should be appreciated that inan alternative embodiment the deformer ring may be rotated with respectto a stationary coiler 5 and a similar effect would be achieved.

An alternative design of energy absorber 31 is shown in FIG. 3. Theenergy absorber 31 is substantially the same as the energy absorber 1with the exception that the circular cover is omitted and the steelstrip 4 is stored as a straight length instead of being coiled aroundthe ring 3.

Other forms of store for the stainless steel strip 4 could be used. Forexample, the strip 4 could be formed into a coil adjacent to, instead ofsurrounding, the ring 3.

In the energy absorbers of FIGS. 1 and 3 the ring 3 is substantiallycontinuous, being broken only by the slot 3 c. This is not essential.The deformer simply provides a defined tortuous path along which thestrip is drawn prior to being wound on a coiler member.

An alternative design of energy absorber 41 is shown in FIGS. 4 and 5.The energy absorber 41 is substantially the same as the energy absorber1 except that the deformer slot 3 c is formed having opposed curved camsurface portions 43 d 43 e defining a serpentine draw through path forthe strip 44. The use of curved deformer surfaces in the embodiments ofFIGS. 1, 3 and 4 provides even plastic deformation rather than highfriction spikes that would result from sharp edges.

In FIG. 4 the outer strip storage annular space 47 is shown containingthe majority of the strip 44, prior to deployment. In FIG. 5 the stripis shown following deployment, wound on the coiler member 45 and fillingthe annular space 48 between the coiler member 45 and the deformer ring43.

In certain embodiments, the annular ring 3 may be replaced by aplurality of spaced apart guide pegs or posts and a deformer structurehaving the required deformer profile. The deformer structure may incertain embodiments be a curved peg or post. Although pegs or posts donot provide continuous inner and outer surfaces the spaced apart innerand outer surfaces of the pegs and deforming structure are able tocontrol the movement of the strip 4 similarly to the inner surface 3 aand outer surface 3 b of the ring 3.

In the described embodiments the stainless steel strip has a constantwidth and thickness. This is usually preferred so that the deploymentcouple is constant along the length of the strip, which is generallyadvantageous in fall arrest devices, as explained above. However, thisis not essential, and if it was desired to have a changing deploymentcouple in a particular application the width, thickness or both of thestrip 4 could be varied.

Further, it is not essential to use a stainless steel strip. Othershapes could be used, for example wires or rods. Other plasticallydeformable materials could also be used, although stainless steel isusually preferred because of its resistance to weathering andpredicable, well understood properties.

The slot and curved surface arrangement used to plastically deform thestrip in the disclosed embodiments is simple and convenient, but notessential. Other deforming mechanisms could be used, for exampledeforming pins or rollers. The number of times the strip is plasticallydeformed as it passes through the deforming mechanism can also bechanged, but if the strip is repeatedly plastically deformed there is arisk of stress failure.

The features of the different embodiments of the invention can beexchanged or combined if desired.

The above described embodiments are examples only and are notexhaustive.

The invention claimed is:
 1. A safety device for use in a fall arrestsystem, comprising: a rotatable drum supporting a safety line wound onthe drum, wherein the drum is constructed to rotate in response todeployment of the safety line from the drum; a speed responsive clutchconnected to the drum, the clutch being responsive to the rotationalspeed of the drum; and a rotational energy absorber operatively coupledto the clutch, the energy absorber including a coiler member, adeforming structure, and a plastically deformable element, theplastically deformable element having a first end coupled to the coilermember, a second free end remote from the first end, and an intermediateportion between the first and second ends positioned through thedeforming structure; wherein the clutch is constructed to couple therotatable drum to the rotational energy absorber in a firstconfiguration when the rotational speed of the drum is equal to orexceeds a predetermined speed, and is constructed to not couple the drumto the rotational energy absorber in a second configuration when therotational speed of the drum is less than the predetermined speed, andwherein in the first configuration, relative rotation of the coilermember and the deforming structure causes the deformable element toplastically deform by being drawn through the deforming structure toabsorb energy.
 2. A safety device according to claim 1, wherein: thedeformable element is not drawn through the deforming structure and thecoiler does not rotate when a torque below a predetermined value isapplied to the coiler member, and when a torque above the predeterminedvalue is applied to the coiler member, the deformable member passesthrough the deforming structure, is plastically deformed, absorbsenergy, and permits rotation of the coiler member and the deformingstructure.
 3. A safety device according to claim 1, wherein: the definesspace about the coiler member which is sufficient to permit only aproportion of the deformable element to be wound on the coiler member.4. A safety device according to claim 1, wherein: the deformable elementis an elongate element, and wherein the coiler member and the deformingstructure define a space between them in which the elongate element isstored after passing through the deforming structure, the space beinginsufficiently large to store all of the elongate element, whereby thespace is filled with the elongate element and the elongate element isstopped from passing through the deforming structure before the free endof the elongate element passes through the deforming structure.
 5. Asafety device according to claim 4, wherein: when the elongate elementhas been stopped from passing through the deforming structure by thespace being filled by the elongate element, the energy absorber willsupport a couple of at least twice a predetermined value.
 6. A safetydevice according to claim 1, wherein: the deformable element is anelongate element having a constant cross section.
 7. A safety deviceaccording to claim 1, wherein: the deformable element is an elongateelement formed as a strip.
 8. A safety device according to claim 7,wherein: the elongate element is a stainless steel strip.
 9. A safetydevice according to claim 1, wherein: the deforming structure includesone or more curved deformation surfaces.
 10. A safety device accordingto claim 9, wherein: the deformable element is an elongate element andthe one or more curved deformation surfaces include two opposed surfaceswhich define a slot for receiving the elongate element such that theelongate element contacts the two opposed surfaces as it is drawnthrough the slot to effect plastic deformation thereof.
 11. A safetydevice according to claim 1, wherein: at least one of the deformingstructure and the coiler member defines a serpentine path for theelongate element to be drawn through to effect plastic deformationthereof.
 12. A safety device according to claim 1, wherein: thedeformable element is formed into a coiled store prior to being wound inthe coil form about the coiler member.
 13. A safety device according toclaim 12, wherein: the coiled store surrounds but is spaced apart fromthe coiler member.
 14. A safety device according to claim 1, wherein:the deformable element is formed into a coiled store about the deformingstructure prior being drawn through the deforming structure.
 15. Asafety device according to claim 12, further comprising: a coversurrounding the coiled store.
 16. The safety device according to claim1, wherein an aperture is formed in the deforming structure, theaperture constructed to plastically deform the deformable element whilethe deformable element passes through the aperture.
 17. The safetydevice according to claim 16, wherein the aperture is a slot having twoopposing sides in contact with the deformable element.
 18. The safetydevice according to claim 17, wherein the deformable structure is formedas a discontinuous ring surrounding the coiler member defining agenerally annular space between the coiler member and the ring, andwherein the ring has two opposed deformation surfaces which define theslot.
 19. The safety device according to claim 18, wherein the slot inthe ring is oriented tangentially to an inner surface of the ring. 20.The safety device according to claim 1, wherein the coiler member andthe deforming structure are constructed to rotate relative to oneanother when a couple transferred by the clutch between the coilermember and the deforming structure is equal to a predetermineddeployment couple that is based on the resistance to plastic deformationof the deformable element.
 21. The safety device according to claim 20,wherein the coiler member and the deforming structure are constructednot to rotate relative to one another when the couple transferred by theclutch between the coiler member and the deforming structure is lessthan the deployment couple.
 22. The safety device according to claim 1,wherein the clutch is constructed to apply a load from the drum to theabsorber as a couple.
 23. The device according to claim 1, wherein therotational energy absorber is different from the speed responsiveclutch.
 24. A safety device for use in a fall arrest system, comprising:a rotatable drum supporting a safety line wound on the drum, wherein thedrum is constructed to rotate in response to deployment of the safetyline from the drum; a speed responsive clutch connected to the drum, theclutch being responsive to the rotational speed of the drum; and arotational energy absorber operably coupled to the clutch, the energyabsorber including a coiler member, a deforming structure, and aplastically deformable element, the plastically deformable elementhaving a first end coupled to the coiler member, a second free endremote from the first end, and an intermediate portion between the firstand second ends positioned through the deforming structure; wherein theclutch is constructed to couple the rotatable drum to the rotationalenergy absorber in a first configuration when the rotational speed ofthe drum is equal to or exceeds a predetermined speed, and isconstructed to not couple the drum to the rotational energy absorber ina second configuration when the rotational speed of the drum is lessthan the predetermined speed, and wherein in the first configuration,the clutch is constructed to drive relative rotation of the coilermember and the deforming structure to plastically deform the deformableelement by being drawn through the deforming structure to absorb energy,and wherein the coiler member and the deforming structure areconstructed to begin to rotate relative to one another when a coupletransferred by the clutch between the coiler member and the deformingstructure is at least equal to a predetermined deployment couple that isbased on the resistance to plastic deformation of the deformableelement, and wherein the couple required to continue relative rotationbetween the coiler member and the deforming structure is equal to thepredetermined deployment couple.
 25. The safety device according toclaim 24, wherein the coiler member and the deforming structure areconstructed not to rotate relative to one another when the coupletransferred by the clutch between the coiler member and the deformingstructure is less than the deployment couple.
 26. The device accordingto claim 24, wherein the predetermined couple corresponds to a maximumsafe load which can be applied to the user.